The field of the invention is magnetic resonance angiography (MRA), and particularly the production of real-time images during dynamic studies.
Contrast-enhanced MRA is becoming a widely used technique for imaging the arterial vasculature. With this technique an MR contrast agent such as Gadolinium-DTPA is injected intravenously as a bolus. Some time later when the contrast bolus arrives in the arteries under study the MR data acquisition is performed, typically with 3D spatial encoding. The technique performs well when compared to conventional x-ray angiography (CA) as a screening method for disease in the carotid and renal arteries as well as the major arteries of the leg and foot.
However, 3D CE-MRA remains inferior to intra-arterial CA with respect to both spatial resolution and temporal resolution. Two main acquisition strategies have evolved in 3D CE-MRA to compensate for these detriments. One strategy is designed to maximize spatial resolution. This is based on the acquisition of a single, high resolution 3D image. The success of this strategy depends on the accurate timing of the arrival of the contrast bolus in the targetted vasculature followed by a 10 to 50 sec long acquisition of 3D data during the xe2x80x9cfirst passxe2x80x9d of injected Gd contrast agent. All acquired phase encodes or xe2x80x9cviewsxe2x80x9d are unique samples of the spatial frequencies of the object, and therefore all views acquired are dedicated to improving the spatial resolution of the final 3D image. A disadvantage of such a high spatial resolution approach is that it requires a long acquisition time and thus no temporally resolved information is provided since every view is dedicated to improving the spatial resolution of the single resultant 3D data set.
The second strategy in 3D MRA is to sacrifice some of the high spatial resolution of the xe2x80x9ctimedxe2x80x9d single acquisition approach and to acquire a series of lower resolution 3D images during the passage of the contrast bolus. The acquisition of 3D images with higher temporal resolution can be done using techniques such as fractional echo and partial NEX k-space acquisition (i.e.  less than 5/8 NEX), or 3D data sets constructed from multiple samplings of several regions of k-space combined with temporal interpolation as described for example in U.S. Pat. No. 5,713,358.
Yet another strategy is to acquire a sequence of 2D images during the passage of the contrast bolus. This strategy is designed to attain both high temporal resolution (approximately 0.5 to 1.0 Hz) and high in-plane spatial resolution of the contrast passage. In one implementation, 2D projection images are acquired repeatedly, generally with complex subtraction, at rates of 1 image/sec or faster. In order to obtain information over an adequately large and useful vascular region, a thick (several cm) slab is generally imaged. However, because the method is 2D, no resolution is provided along the slab select direction itself. Despite this loss of information along the slab direction, the temporally resolved information provided by such 2D CE-MRA techniques can identify anomalies in contrast enhancement that have functional significance. In a variation of this, the projection angle can be altered or cycled during the acquisition.
What is desirable is a technique which preserves the high spatial resolution in all three dimensions of the single, bolus-timed 3D approach but can still provide dynamic information at the high temporal resolution of the 2D methods.
The invention comprises an MR imaging technique that makes it possible to acquire high temporal resolution 2D images simultaneous with the acquisition of a high spatial resolution three-dimensional (3D) data set. This is done by including or xe2x80x9cembeddingxe2x80x9d the acquisition of data for 2D imaging within a 3D imaging scan. This invention has particular application to contrast enhanced MR angiography (CE-MRA) where both temporally resolved images capturing the contrast passage through the vascular system and high spatial resolution 3D images are desirable. The invention has further applications when the 2D image sequence is reconstructed in real time. In this case information from the 2D images can be used to modify the 3D acquisition in real-time or to initiate or modify some other process.
Another aspect of the invention is to provide a seamless transition between the acquisition of 2D images and the acquisition of a 3D image. These include fluoroscopically triggered CE-MRA, interventional MR techniques, and moving table-top CE-MRA. This is accomplished by using the same pulse sequence throughout the procedure and exciting the same FOV such that magnetization equilibrium is not disturbed during the transition.